Permanent magnet generator control

ABSTRACT

A method of and a system for controlling a permanent magnet AC generator ( 10 ), wherein the generator is provided with stator windings and permanent magnets in the rotor and wherein the generator is connected to a drive unit ( 50 ), wherein the generator is further provided with a semiconductor converter provided with AC output connected to the generator output and a DC link for controlling the output voltage of the generator, and the converter is further provided with filter means for filtering the output of the converter so that a filtered output of the converter is fed to the generator output.

FIELD OF THE INVENTION

The present invention relates to a permanent magnet synchronous generator control.

Especially the present invention relates to a method of controlling a permanent magnet synchronous generator and a system for controlling a permanent magnet synchronous generator. Preferably the present invention is used as a ship's auxiliary power supply or corresponding power supply.

PRIOR ART

Motor-generator sets, typically diesel engine-synchronous generator sets, are well known equipments for power supply on ships, especially as auxiliary power supplies, UPS (Uninterruptible Power Supply) and so on. The synchronous generator is generally provided with external excitation. The generator output voltage and frequency have to be constant. The frequency is controlled via motor speed, (motor revolution), and in this way the active generator power is controlled if the generator is connected to the AC voltage grid. Generator output and load voltage control are available due to excitation control via generator external excitation.

In the case of the short circuit on the load side, the generator has according to safety regulations to able to produce minimum three times higher current as the rated one.

Permanent magnet (PM) generators can be used instead of synchronous generators with external excitation. When permanent magnet generators are used, no external excitation and thus no external excitation power is needed, and thus the generator efficiency is higher than the generator efficiency of traditional generators. The disadvantage of the permanent magnet generators is that permanent magnet excitation can not be controlled, and therefore the generator output voltage and the load voltage cannot be controlled so that they remain constant.

SUMMARY OF THE PRESENT INVENTION

The object of the present invention is to eliminate the drawbacks of prior-art solutions and to achieve permanent magnet generator control that will make it possible to control the generator output voltage.

Another object of the present invention is to achieve a system wherein the rating and size of the permanent magnet generator may be minimized.

A further object of the present invention is to achieve a system wherein the number of the components in the control circuit can be minimized and the electrical circuit may be done as simple as possible.

In the present invention the synchronous permanent magnet generator control is improved the use of an additional three phase power converter consisting of a semiconductor switch bridge connected to the output of the generator and a DC capacitor coupled to the converter DC circuit. An output filter may be connected to the generator output in order to achieve sinusoidal converter output voltage.

The present invention is in detail defined in the attached claims.

The rated converter power is related to the PM generator construction. Such construction has benefits compared with traditional solution, especially the lower weight and size of the generator and higher efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred embodiments of the present invention will be described in detail by reference to the drawings, wherein

FIG. 1 presents the PM generator voltage control via additional reactive power converter,

FIG. 2 is a PM generator simplified equivalent circuit,

FIG. 3 presents the connection, wherein the converter is connected on the tap winding of the pm generator,

FIG. 4 presents a simplified filter construction, and

FIG. 5 is a preferred circuit for PM generator and converter for voltage control according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 presents a diesel engine-permanent magnet generator apparatus for a ship's auxiliary power supply producing a three phase 450 V AC output voltage U_(U), U_(V), U_(W) . The apparatus comprises a three phase synchronous PM generator 10 with stator windings and a permanent magnet rotor and providing a three phase output voltage. The generator is connected via a common shaft 60 to a diesel engine 50 operating as a drive unit for the generator.

The PM generator apparatus is further provided with an additional three phase power converter controlling the PM generator output voltage. Its AC output is connected to the PM generator output GENOUT and consists of a full wave converter bridge 30 and a control unit 40. A DC link capacitor C_(DC) is coupled on the converter DC side. The apparatus is further provided with a converter output filter at the converter AC output for filtering the converter three phase AC output voltage in order to provide a sinusoidal voltage and current. The DC side has no other connections than the DC link capacitor.

The converter bridge is a full-wave bridge with pulse-width-modulated semiconductor switches V1 to V6, such as IGBTs, in upper and lower arms and flywheel diodes D1 to D6 connected in inverse-parallel with the semiconductor switches. The semiconductor switches V1 to V6 are controlled with pulse-width modulation by means of a PWM control in the control unit 40.

The filter is a three phase choke-capacitance filter structure L1, Cf, L2 having a serial coupling of chokes L1 and L2 and the capacitor unit coupled between them in each phase. The filtering can also be based only on L1 choke, without L2 and Cf. The converter can produce, take and control the reactive power from the generator by means of the control unit 40.

In this way the generator output voltage and the load voltage U_(U), U_(V), U_(W) can be controlled and kept constant during the different load conditions. Rated converter power is related to the PM generator construction, and it is one fraction of the rated generator power, in the range of 30% to 50%. Such construction has the benefits compared with traditional solution (synchronous generator with external excitation), like lower generator weight and size and higher efficiency.

The circuit according to the present invention can have additional benefits if additional control strategy is applied.

Short circuit current is by permanent magnet generator limited by the relative high internal stator inductances. Therefore the demand of three time short circuit current in the ratio to the rated current is very relevant to the PM generator production costs. The short circuit generator current can be reduced, (for example on two times rated current), when the additional converter circuit is controlled to supply current at short circuit conditions, in the 3. quadrant, (reactive current), up to value of the generator rated current. In that case, the load short circuit current will be three times rated current, and the generator short circuit current will be only two times rated current. (Load current=generator current+converter current).

That kind of additional converter operation feature makes it possible to achieve a compact PM generator construction.

For the converter operation at short circuit conditions on load side it is needed to have some active power for the covering of converter power losses. The power source for such active power can be a fraction of remained load voltage at short circuit conditions, (for example 10% of the generator voltage.) The other solution will be from the DC link capacitor, which can be dimensioned for power losses recovering for the time of short circuit duration.

Taking in account the PM generator's simplified one phase equivalent circuit, FIG. 2 consists of the generator's winding LGEN and the EMS (electromotoric source). The total circuit can be significant simplified. If we take in the PM generator winding one tap per phase, we will have the circuit like in FIG. 3. In FIG. 3 the converter is connected to the tap TAP1 in the PM generator winding.

In this way, even at short circuit at the generator terminals, (zero voltage at load and generator), tap voltage will be minimum 10% if the tap is in the position 9:1 of the internal inductance. At the same time, necessary converter filter choke will be integrated in the three generator windings LG1, LG2, LG3 in each phase, FIG. 4, with the tap between the second and third windings LG2 and LG3. Here the winding LG3 replaces the choke L2 in FIG. 1.

If the remained L2 inductions value is high enough, the circuit can be significant simplified, see FIG. 5, where the filter is not present at all.

It is obvious to the person skilled in the art that the embodiments of the invention are not restricted to the examples presented above, but that they can be varied within the scope of the following claims. Besides IGBTs, the fully controllable semiconductor switches used may also consist of other fully grid-controlled semiconductor switches, i.e. switches that can be turned on and off, such as transistors or MOSFETs. 

1. A method of controlling a permanent magnet AC generator (10), wherein the generator is provided with stator windings and permanent magnets in the rotor and wherein the generator is connected to a drive unit (50), characterized in that the generator is further provided with a semiconductor converter provided with AC output connected to the generator output and a DC link for controlling the output voltage of the generator, and the converter is further provided with filter means for filtering the output of the converter so that a filtered output of the converter is fed to the generator output.
 2. The method as defined in claim 1, characterized in that the control converter circuit is controlled to supply current at short circuit conditions in addition to the generator short circuit current supply.
 3. The method as defined in claim 1, characterized in that at short circuit conditions active power on the load side for the covering of converter power losses is attained from the remained load voltage.
 4. The method as defined in claim 1, characterized in that A DC link capacitor (C_(DC)) is coupled on the converter DC side, and that at short circuit conditions active power on the load side for the covering of converter power losses is from the DC link capacitor which is dimensioned for power losses recovering for the time of short circuit duration.
 5. A system for controlling a permanent magnet AC generator (10), wherein the generator is provided with stator windings and permanent magnets in the rotor and wherein the generator is connected to a drive unit (50), characterized in that the generator is further provided with a semiconductor converter provided with AC output connected to the generator output and a DC link for controlling the output voltage of the generator, and the converter is further provided with filter means for filtering the output of the converter so that a filtered output of the converter is fed to the generator output.
 6. The system as defined in claim 5, characterized in that the control converter circuit is controlled to supply current at short circuit conditions in addition to the generator short circuit current supply.
 7. The system as defined in claim 5, characterized in that the filter is a choke-capacitance filter structure (L1, Cf, L2).
 8. The system as defined in claim 5, characterized in that the filtering circuit comprises only on L1 choke.
 9. The system as defined in claim 5, characterized in that converter filter choke coupling is integrated in the generator windings (LG1, LG2, LG3) with a tap (TAP1) between the windings.
 10. The system as defined in claim 5, characterized in that converter filter is totally integrated to the generator windings with no external filter means at the generator output.
 11. The system as defined in claim 5, characterized in that the generator is a multiphase, preferably a three phase, PM generator with three phase stator windings.
 12. The system as defined in claim 5, characterized in that the generator is a generator in a ship's auxiliary power supply or a corresponding power supply connected to a ship's engine, such as a diesel engine.
 13. The system as defined in claim 5, characterized in that the converter bridge is a full-wave bridge with semiconductor switches (V1 to V6), and preferably with flywheel diodes (D1 to D6) connected in inverse-parallel with the semiconductor switches, and that the semiconductor switches V1 to V6 are controlled by means of a control unit (40).
 14. The system as defined in claim 9, characterized in that converter filter is totally integrated to the generator windings with no external filter means at the generator output. 